This is among my favorite anime, I think it was one of my first exposures to an expanded universe.
The Concept of several anime series all existing in the same universe and tied into one another was fascinating to me.

Accelerator was one of my favs, probably because I liked how he was presented to be so powerful yet he seemed to change as the story progressed. In a way he kinda reminds me of Hyoubu Kyousuke from Zettai Karen Children. The way he is presented as a powerful antagonist but has a lot of potential as a protag.

Unfortunately I fell behind on the series, Didn't see much beyond index II and railgun II.
I would need a roadmap to figure out what order to rewatch everything again....